A mortise lock is provided which includes a casing having a front plate for confronting a door frame and a pair of opposed side walls, the front plate having an opening for a latch bolt. A latch bolt is movable with respect to the casing between an extended position and a retracted position by a linkage connected to a hub to receive a door handle shaft, the latch bolt is rotatable axially, and the hub is rotatably secured between the opposed side walls of the casing. A slider plate for initiating movement of the linkage is provided that is movable from a first position wherein the linkage moves the latch bolt to the retracted position to a second position wherein the linkage moves the latch bolt back to the extended position. The slider plate has a preset adjuster, adjustable through at least one access aperture in the casing. An adjustable spring is included to urge the slider plate in a direction away from the hub to cause the slider plate to move to the second position and an access aperture is included in the casing to facilitate access to the adjustable spring.